Optical modulators have been employed for many years in the field of optical communications to accept modulated data in electrical format (typically radio frequency or RF) and transfer the data onto an optical carrier. In a Mach-Zehnder optical modulator 20, the main components of which being generally shown in FIG. 1A (PRIOR ART), a beamsplitter 22 divides the laser light from an input optical waveguide 24 into two optical beams propagating in parallel waveguides defining optical paths 28A and 28B, at least one of which having a phase modulator in which the refractive index is a function of the strength of the locally applied electric field. In the example of FIG. 1A light in both optical paths 28A, 28B undergoes a phase modulation, although in other configurations the refractive index in only one of the optical paths could be modulated with respect to the other. The beams are then recombined by an output optical combiner 26. Changing the electric field on the phase modulating paths determines whether the two beams interfere constructively or destructively when recombined, and thereby controls the amplitude or intensity of the exiting light. In some configurations, the phase of the exiting light can be controlled via a variety of means such as by manipulating the phase modulation signal, or through design.
In the configuration shown in FIG. 1A, the modulating electric field is provided by a segmented travelling wave electrode 21 (or TWE) that consists of two or more transmission line conductors 30A, 30B oriented substantially parallel to the optical paths 28A, 28B, and a plurality of pairs of waveguide electrodes 32A, 32B. At least one of the transmission line conductors 30A and 30B carries the RF signal, while the other may either also carry the RF signal or be connected to ground. Each waveguide electrode 32A, 32B is connected one of the transmission line conductors 30A, 30B via a corresponding tap or bridge conductor 34A, and 34B. Each bridge conductor 34A, 34B branches out of one of the transmission line conductors 30A, 30B in a direction substantially perpendicular to the optical path 28A, 28B. The transmission line conductors 30A, 30B convey an RF signal along an RF path that is substantially parallel to the optical paths 28A, 28B.
The configuration shown in FIG. 1A is known as a Mach-Zehnder modulator operated in “push-pull” mode is referred to as a series push-pull travelling wave electrode, after Klein et al., “1.55 μm Mach-Zehnder Modulators on InP for optical 40/80 Gbit/s transmission networks”, OFC/NFOEC 2006, paper TuA2, and described in further detail by R. G. Walker, “High-Speed III-V Semiconductor Intensity Modulators”, IEEE J. Quant. Elect., vol. 27(3), pp. 654-667, 1991. In a series push-pull configuration, a single voltage signal or field is used to phase modulate the interfering signals in the two arms in anti-phase. Each pair of waveguide electrodes 32A, 32B, as shown in FIG. 1A, impart a phase change to the optical wave in the waveguide 28A, 28B and also act as a pair of capacitors in series and as a load on the main transmission line conductors 30A, 30B.
A travelling wave electrode Mach-Zehnder optical modulator can be driven using a single RF signal input, as illustrated in FIG. 1B (PRIOR ART). In the illustrated example, the travelling wave electrode 21 of the modulator 20 includes a first transmission line conductor conveying the input electrical signal, therefore acting as a signal transmission line conductor (S), and a second transmission line conductor connected to a ground reference, therefore acting as a ground transmission line conductor (G). This modulator configuration is single-end as it includes a single signal transmission line and is sometimes referred to as an SG modulator (also known as coplanar strip). In the specific embodiment shown in FIG. 1B, the electrical modulation signal is provided by an RF voltage source 50 having a single signal line 52 and a ground line 54, both embodied by a RF waveguide such as a coaxial cable. The signal line 52 of the driver 50 is connected to the signal transmission line conductor S of the travelling-wave electrode 21, whereas the ground line 54 of the driver 50 is connected to the ground transmission line conductor G of the travelling-wave electrode 21. A nominal terminal load 56 (e.g., 50 ohms) joins the distal ends of the S and G transmission lines. The modulation voltage across the arms of the travelling wave electrode is the difference between the signal voltage and ground.
It should be noted that other types of RF drives are known in the optical telecommunications industry, requiring other arrangements of transmission line conductors in the modulator. For example, the prior art includes optical modulators with differential-drive GSGSG and GSSG formats.
It is a well-known objective of electronics and photonics to reduce size. In integrated electronics and photonics, size usually refers to the footprint in the XY plane, i.e., the area occupied by a device at or near the surface of a semiconductor wafer. Integrated Mach-Zehnder optical modulators are typically much longer than they are wide (i.e., larger in the X direction than they are in the Y direction, according to the convention shown in FIGS. 1A and 1B). For example, high bandwidth modulators commonly used for digital optical fiber communications, made of indium phosphide or lithium niobate, can be ten times longer than they are wide, in large part due to the length of interaction required between the electrical and optical signals for optimal performance. As a result of this extreme aspect ratio, much attention is focused on reducing the length of such devices. However, an increase in the number of devices per chip or per wafer can also be achieved by: (1) reducing the spatial separation between side-by-side neighboring devices in the Y direction; and (2) reducing the width of the individual device itself.
In the prior art modulator design shown in FIG. 1B, compacting the space between devices has serious drawbacks. Although one of the transmission line conductors is grounded in this design, the other carries the signal, and is sometimes known as the “hot” or “live” conductor. At high frequencies in particular, the capacitance and inductance of this signal transmission line conductor is sensitive to the surrounding environment, i.e., to the characteristics and geometry of dielectrics and other conductors nearby. If other devices are placed close to it on the same chip, the performance of the device can be largely impaired. Furthermore, the presence of conductors in neighboring devices can cause them to be electromagnetically coupled to the signal conductor of the Mach-Zehnder modulator, and a disadvantageous phenomenon known as cross-talk can occur. Even if the Mach-Zehnder modulator is the only device on a chip, the presence of a cleaved edge of the chip too close to the signal conductor can be problematic, such as would be the case if the modulators are tightly packed on a wafer before dicing.
A GSSG configuration offers some improvement with regard to electrical isolation from the external environment. Since the outer two transmission line conductors are electrically connected to ground with the pair of signal transmission line conductors nested between them, this type of Mach-Zehnder modulator can be placed very close, in the Y direction, to another device and yet not electromagnetically couple to it. However, the improvement in electromagnetic isolation comes at the cost of a wider device, since four transmission line conductors are required, versus only two for the device shown in FIG. 1B.
Both these designs have drawbacks with regard to reducing device width. Both designs rely on at least one transmission line conductor that has a good path to ground. For good performance, this conductor should be very wide, so that current returning from the signal transmission line conductor encounters little inductance. Making the ground transmission line conductor too narrow can cause unintentional and disadvantageous degradation to the fidelity of the signal. Obviously, the desire for a narrow modulator is at odds with the need for a wide transmission line conductor for ground.
There remains a need, therefore, to provide designs for a travelling wave electrode Mach-Zehnder modulator that can be made narrow and placed in close proximity to other devices on the same chip. Preferably, such designs should avoid or minimize degradation of the performance of the modulator caused by: (1) interaction between the signal conductor(s) and the environment; (2) interaction between the signal conductor(s) and neighboring devices; or (3) a grounded transmission line conductor that is too narrow to provide a sufficient current-return path.